How to eliminate brassiness by using a semi perman

Hi!
Here in Greece, where I am living, I can't find a toner to remove brassiness from my hair. My natural hair are level 6, with blonde highlights (doing in a salon). I dyed my hair (in a salon) with 9 and 9.3 wella koleston perfect, ratio 1:1, adding 30 developer applied at roots for 30 minutes and then in the rest of the head for only 10 minutes the result was a blonde hair with highlights but had brass tones. It was a nice uniform colour but I wanted something less warm. So after 40 days I used a formula of 10/1, 9/03, ratio 2:1 with 30 developer applied roots and lengths starting from back of the head for 30 minutes, (applied at home with a friend). The result was a less warm blonde but still there is brassiness. I read that I could use a semi permanent dye as a toner to reduce brassiness and the color for level 9 hair is violet. So is it ok to use a semi permanent violet dye without ammonia or it is going to turn my bleached highlights purple? I will do a strand test off course first.
Also, a friend of mine (6 natural base) uses ash tone and purple tone permanent dye, ratio 1:1, with 30 developer for only 5 minutes to touch up hair roots and tone her hair between hair saloon visits (she does blonde highlights) to minimize the damage in her hair. So which is the less damaged solution for someone to touch up the roots at home between hair salon visits making blonde highlights?
Note that my hair is in good condition.

I would suggest finding a Purple shampoo. It will do the job very well to tone down the brassiness. You don't want to go and take the chance of ruining your salon hair job from a boxed dye!! If you have John Frieda where you live they have great hair products for blondes. Also you can try good old Lemon juice mixed with a bit of conditioner and sit in the sun.